Vinyl chloride is produced commercially by the pyrolysis of 1,2-dichloroethane. The rate of conversion increases with the temperature and, the higher the temperature, the more undesired byproducts are generated during the cracking process. Following the separation of hydrogen chloride and vinyl chloride and the simultaneous extraction of traces of low-boiling byproducts such as 1,3-butadiene, acetylene, monovinyl-acetylene and methyl chloride, the 1,2-dichloroethane which has not been reacted during the cracking process still contains 1,1-dichloroethylene, chloroform, carbon tetrachloride, trichloroethylene, benzene and, above all, substantial quantities of 2-chlorobutadiene-1,3 and some 1-chlorobutadiene-1,3. In recycling the unreacted 1,2-dichloroethane, the elimination of these byproducts has been found to be necessary to prevent the plugging of the cracking furnace with soot and coke.
The removal of the above-mentioned byproducts by distillation has the drawback of resulting in the accumulation and subsequent polymerization of the chloroprene in the distillation column, causing operational difficulties.
From U.S. Pat. No. 3,876,714, it is generally known that, prior to the separation of the vinyl chloride and hydrogen chloride from the unconverted dichloroethane small amounts of chlorine can be added to the cracked gas containing the 1,2-dichloroethane. Following the addition of the chlorine, the liquid reaction mixture must be kept for a certain period of time in a storage tank before it can be further processed. Aside from the fact that this method does not eliminate all of the previously mentioned problems, it requires that in order to eliminate the danger of corrosion, special preventive measures be taken or that apparatus made of extremely fine and very expensive materials be employed.
A process is disclosed in U.S. Pat. No. 3,920,761 in which 0.01 to 1.0 weight % chlorine (based on the weight of the dichloroethane originally used) is added, dissolved in 1,2-dichloroethane, prior to the separation of vinyl chloride. Under the prevailing conditions, a portion of the chlorine reacts with the vinyl chloride to produce valueless 1,1,2-trichloroethane and a portion of the unreacted dichloroethane is partially substituted. Thus, a quantitative chlorination of chloroprene cannot be observed.
According to German Pat. No. 2,416,786, during the processing of the reaction products of a thermal cracking of 1,2-dichloroethane, the part of the column in which the high boiling products are to be separated is flushed with chlorine gas without a catalyst being present, after the hydrogen chloride and the vinyl chloride have been separated. The resulting decrease in chloroprene extends the time span over which the cracking furnace and the column may be used; however, a quantitative extraction of chloroprene, which is necessary in order to extend the time span over which the cracking furnace may be utilized, can still not be achieved. The simultaneous presence of chlorine in the distillation bottoms at temperatures of about 100.degree. to 110.degree. C. results in a noticeable chlorine substitution of the 1,2-dichloroethane, generating high-boiling 1,1,2-trichloroethane and is thus detrimental for economic reasons.
Consequently, the purpose of this invention is to provide a process for treating unreacted 1,2-dichloroethane from 1,2-dichloroethane cracking, prior to recycling, in which troublesome byproducts, especially 1,2-chlorobutadiene-1,3 and 2-chlorobutadiene-1,3 are eliminated in a simple and suitable manner while avoiding the drawbacks of the previously known methods and while increasing the period of time over which the pyrolysis reactor and the column can be operated.